Male infertility is a concern for many couples. Worldwide studies have shown that 2%-7% of all couples have experienced difficulty in achieving conception or are unable to bear children, especially as they near the end of their reproductive life. (Sara A., Advances in Fertility and Sterility, Proc. Twelfth World Congress Fer. Steril. 4:91-92 (1987)). Furthermore, among men who seek help or advice at fertility clinics, slightly more than 10% are diagnosed as having oligospermia or azoospermia of unknown origin. (Hargreave, T. B., The Management of Male Infertility, T. B. Hargreave and T. E. Soon, eds. (Singapore: PG Publishing, pp. 2-21, 1990)). At this time, little is known about the causes of reduced spermatogenesis and, although various treatments are available, improved methods are still needed.
XY sex chromosomes are found in a multitude of species throughout the animal kingdom. It is thought that XY chromosomes arose independently in many evolutionary lineages, in each case deriving from an ordinary autosomal pair. According to prevailing theories, once recombination between nascent X and Y chromosomes becomes restricted, the gene content of the Y chromosome declines steadily and inexorably. Degeneration of the Y chromosome is well documented in Drosophila (Rice, Science 263:230-232 (1994); Steinemann & Steinemann, Proc Natl. Acad. Sci. USA 89:7591-7595 (1992)), and has been shown to be an ongoing process even among mammals, which are generally considered to exhibit extreme differentiation of the X and Y chromosomes. The few genes that persist on highly differentiated Y chromosomes are thought to be relics of this common ancestry with the X chromosome. According to this view, Y-chromosomal genes were once, or still are, shared with the X chromosome (Rice, BioScience 46:331-343 (1996)).